Ettekanded / Talks

Danel Ahman: When is a Container a Comonad?

Abstract:
We present a specialization of containers to capture the common
situation in programming where every position in a datastructure
determines a sub-datastructure rooted by that position. Some natural
examples of such datastructures are non-empty lists and
node-labelled trees, and datastructures with a designated position or
focus (zippers). Containers, as introduced by Abbott, Altenkirch
and Ghani are a neat representation for a wide class of parameterized
datatypes (set functors) in terms of a set of shapes and a set
of positions in each shape. They cover lists, streams, various kinds
of trees, etc. and can be used as a "syntax" for programming
with these datatypes and reasoning about them.

We show that directed containers are no less neat than containers.
While containers denote set functors via a fully-faithful functor,
directed containers interpret fully-faithfully into comonads. Our main
result is that every comonad whose underlying functor is the
interpretation of a container is the interpretation of a directed
container. So the answer to the question in the title of this talk is:
a container is a comonad exactly when it is a directed container.

In this talk I will cover the basics of containers, introduce the
notion of directed containers, show some motivating examples and
summarize the main results of our work. (Joint work with James
Chapman and Tarmo Uustalu, to be presented at FoSSaCS 2012)

Anna Aljanaki: Trajectory Clustering on GPS Data

Abstract:
One of important spatio-temporal data mining tasks is grouping similar
trajectories in order to discover frequent patterns in tracked object
motion. Frequent trajectory mining has many applications, such as
hurricane prediction, city traffic monitoring, animal behaviour
explanation. In this talk we will handle this subject as well as
developing similarity measures for routes.

Dan Bogdanov: Building a universal secure computation platform
based on protection domains

Abstract:
Sharemind is a secure data analysis system based on secret sharing and
secure multi-party computation. Given the recent breakthroughs in
alternative secure computation techniques like fully homomorphic
encryption and driven by the wish to quickly implement new computation
protocols, we have upgraded the design of the Sharemind machine and
programming languages. This talk will share some of the ideas that
inspired the design of Sharemind 3.

Rafik Chaabouni: Set membership and range proofs

Abstract:
Set membership and range proofs are cryptographic building blocks.
Both of these building blocks are Zero-Knowledge proofs. The goal of
these proofs is to convince some verifier on the veracity of a
statement without revealing any more information besides the validity
of the statement. For the case of set membership, the statement is
linked to a secret value that a prover has chosen in a public set. His
objective is to prove that his secret is indeed in the public set
without revealing any more information besides its membership to the
set. Range proofs are particular cases of set membership, where the
public set is an integer interval.

Set membership and range proofs have several applications. Some of
the most important ones are related to the ITU's Child Online
Protection (COP) Initiative. Indeed, range proofs (when considering
age intervals) can bring a solution to protect children on internet by
two means. First, range proofs can be used to forbid children to
access adult contents. Secondly, range proofs can protect children
from adult predators, by keeping them away from youth's platform such
as forums and social networking services.

James Chapman: Relative Monads and Lambda Calculus

Abstract:
I will describe the relative monad structure of the untyped/typed
lambda-terms which captures the notion of parallel substitution.
Interestingly, the algebras for these monads are models
(evaluators/interpreters/semantics) of the lambda-terms. I will
explain the machinery of relative monads and their algebras, and go
through the implementations of the lambda-term examples in the
programming language Agda.

Denis Firsov: Certified parsing

Abstract:
Parsing is a fundamental component of many software pieces. The
ultimate goal of our project is to implement a parser generator for
context-free languages, which, given a context-free grammar, should be
able to provide a parser together with an independently checkable
certificate of its correctness.

As a first approximation of our main goal, we implemented a parser
generator for regular languages and showed its correctness using the
Agda programming language. Specifically, we programmed a
transformation
of regular expressions into a Boolean-matrix based representation of
finite automata. And we proved (in Agda) that a string matches a
regular expression if and only if the finite automaton accepts it.

Rusins Freivalds: Quantum finite automata on infinite words

Abstract:
The definition of language recognition by quantum finite automata in
the case when the language consists of infinite words is somewhat
non-trivial because there is no easy way how to consider a measurement
after processing an infinite word. However a natural definition is
possible based on measure theory developed by E.Borel and the standard
definition of language recognition by deterministic and
nondeterministic finite automata in the case when the language
consists of infinite words developed by R.Buchi.

We prove that Measure-Many finite quantum automata can recognize
with probability 1 languages not recognizable by deterministic and
nondeterministic finite automata. On the other hand, Measure-Once
finite quantum automata can recognize with a bounded error only
languages recognizable by deterministic finite automata.

Liina Kamm: How to make proving cryptographic protocols less
tedious

Abstract:
ProveIt is a semi-automatic prover tool that helps the user make
game-based cryptographic proofs by checking whether the specified
reduction schema is applicable for the statement in question. The tool
is given the formalisation of a security property through a short
program also called a game. The user can apply different reduction
schemas to the game that is then rewritten according to the applied
schema and a new game is obtained. This kind of reduction process
assures that the resulting proof is sound. The tool does not perform
automatic proofs but rather assists the user in the proof process.
This kind of assistance is necessary to help make the tedious and
tricky proof validation tractable.

Peeter Laud: The design of SecreC for Sharemind 3

Abstract:
SecreC is the high-level programming language for expressing
privacy-preserving algorithms in the Sharemind framework. The new
features of the framework's third version also inspired an overhaul of
the language. In this talk, we describe the language and its
semantics, and justify the design decisions we made. In Sharemind 3,
SecreC will be an imperative language that can polymorphically use
various sets of secure computation algorithms. Additionally, the
programs in this language satisfy certain information flow security
properties.

Sven Laur: Secret sharing and arcs from projective geometry

Abstract:
Linear secret sharing is one of the cornerstones of secure
multiparty computation. It is common to consider the case where
the secret and its shares are field elements. Although this
assumption gives a great leverage and allows us to prove many
important results, it is also quite restrictive---most common
data types used in hardware design and programming are residue
rings. In this talk, we discuss which claims still hold if we
omit this assumption. In particular, we study a surprising
trinity consisting of threshold secret sharing, maximal distance
separable codes and arcs. For finite fields, all three terms
describe different facets of the same combinatorial object. We
show how to extend these definitions to rings without loosing the
cherished correspondence.

Abstract:
In 2010, Groth constructed the only previously known
sublinear-communication NIZK circuit satisfiability argument in the
common reference string model. We optimize Groth's argument by, in
particular, reducing both the CRS length and the prover's
computational complexity from quadratic to quasilinear in the circuit
size. We also use a (presumably) weaker security assumption, and have
tighter security reductions. Our main contribution is to show that the
complexity of Groth's basic arguments is dominated by the quadratic
number of monomials in certain polynomials. We collapse the number of
monomials to quasilinear by using a recent construction of
progression-free sets.

Keiko Nakata: On streams that are finitely red

Abstract:
In this talk, I will examine alternative definitions of streams being
finitely red. These definitions are organized into a hierarchy
including also some well-known alternatives in intuitionistic
analysis. The hierarchy collapses classically, but is
intuitionistically of strictly decreasing strength. The differences
in strength are characterized in a precise way by weak instances of
the Law of Excluded Middle.
(Joint work with Marc Bezem and Tarmo Uustalu)

Alejandro Russo: Information-flow security via libraries

Abstract:
Special purpose languages have been developed over the years to
guarantee confidentiality or integrity of data. In practice, the
impact of these languages has been limited. Rather than producing new
languages from scratch, it has been shown that confidentiality and
integrity policies can be also guaranteed via a library, which makes
this technology more likely to be adopted. This talk seeks to describe
the principles behind security libraries to provide confidentiality
and integrity of data in Haskell. The talk will also present the
state-of-the-art on security libraries and the main challenges ahead
to take this technology into practice. The talk is based on a series
of recently published papers.

Andri Saar: Compiling ABS to Scala

Abstract:
ABS is a modelling language that is designed to be executable. We are
working on a new back-end for ABS, utilizing the programming language
Scala, that would allow true distributed execution of ABS models. The
concurrency model of ABS is two-layer: we have concurrent object
groups which communicate with only asynchronous messages and futures
(reminiscent of actors) and inside those object groups we have
individual objects which use cooperative multitasking. This is very
different from the concurrency model of Scala, and thus poses some
interesting problems for our compiler. We present the first steps in
formalising our approach to compiling ABS code to Scala: we look at
the cooperative multitasking inside one COG and show
how that can be implemented using delimited continuations, and proceed
to show the correctness of this approach.

Abstract:
We talk about the Advanced Encryption Standard (AES) block cipher
implementation on the Sharemind framework. Sharemind is a practical
secure multi-party computation framework based on secret sharing. In
our AES implementation both plaintext and cipher keys are secret
shared among many computing parties. We give an overview of the
parallelization techniques used together with the corresponding
benchmarking results.

Elmo Todurov: A finer module system and separate compilation for
ABS

Abstract:
The ABS language has a Haskell-style module system. It is
possible to specify names of types and functions to be exported, but
not types. Therefore it is impossible to typecheck each module
separately. In addition, in the ABS language there are no abstract
types (also known as opaque types), which are useful for hiding
implementation.

In my master's thesis, a module system (supporting separate
compilation and abstract types) and a language based on ABS is
designed and a type system given for it. The language has explicitly
typed imports and exports, sufficient for typechecking each module
individually. Safety is then proved for the type system.

This talk presents the key points of my master's thesis:
background on ABS separate compilation,
overview of our work,
a brief overview of our type system,
a brief glimpse into the soundness proof.

Abstract:
We defined a new notion of computational indistinguishability:
termination-insensitive computational indistinguishability
(tic-indistinguishability). Tic-indistinguishability models
indistinguishability with respect to distinguishers that cannot
distinguish between termination and non-termination.

We sketch how the new notion allows to get computational soundness
results of symbolic models for equivalence-based security properties
(such as anonymity) for processes that contain loops, solving an open
problem.

Tarmo Uustalu: Context-dependent notions of computation and
comodels of theories

Abstract:
Multiple notions of context-dependent computation, typically
consisting in transformation of a homogeneous datastructure based on
local rules applied uniformly to every node, are neatly analyzable in
terms of comonads. Some examples include cellular automata execution,
dataflow computation and attribute evaluation on trees. In this talk,
I show that these examples also admit a finer analysis in terms of
comodels of theories. This analysis explicitly dissects a notion of
context into operations for observing contexts and equations they must
satisfy. These operations make natural primitives for the
corresponding languages of programming. But also natural primitives
for these languages are combinators for generating/switching contexts,
sending correctly constructed comodels to the unique homomorphism to
the cofree comodel.

Needless to say, this is an attempt to dualize the tale of effectful
notions of computation, monads, models of theories and effect
constructing operations, incl. effect handlers à la Plotkin and
Pretnar. Comodels were used by Power and Shkaravska's in an analysis
of arrays.